1. Field of the Invention
This invention relates to an energy beam exposure system for exposing lithographic resist by using a beam which is scanned across the resist, and more particularly it relates to a system for analysis of heating produced by scanning and providing modified scanning patterns for avoiding overheating of the resist.
2. Description of related art.
A system known as the Hontas (EL3) type of E-beam exposure system for writing patterns on resist writes lithographic patterns in the resist with a shaped electron beam exposure system which minimizes the time wasted by workpiece positional requirements. The writing field is pieced together by sequential exposure of an array of rectangular sub-fields written in a raster sequence. An early version of the Hontas system of E-beam exposure is described in U.S. Pat. No. 3,900,736 of Michael et al for "Method and Apparatus for Positioning a Beam of Charged Particles" (commonly assigned.) The Hontas system employs a step-and-repeat method of performing the tasks of registration, writing and transporting of the workpiece to expose each field of a multi-field pattern on a workpiece. In the E-beam writing process employed in the Hontas system, the work table is in continuous mechanical motion in the X-Y plane beneath the E-beam. The motion is generally provided along a serpentine, boustrophedontic path, as the oxen plow from one end of a column to the next column along the "Y" axis. Thus, the continuous motion occurs along one axis herein defined as the "Y" axis, whereas the motion at right angles to the X axis, at the end of each column, occurs as a step function at the end of each Y axis excursion in the direct-ion referred to herein as the "X" axis.
An E-beam source produces a beam which is shaped into various spot shapes and blanked by a deflection and aperture apparatus under the control of spot shaping apparatus and blanking apparatus as in Michael et al U.S. Pat. No. 3,900,736 cited above. The positioned and shaped spot is controlled, in part, by spot shaping, and a blanking analog electronics unit under the control of digital electronics in pattern control section. This determines the pattern written in the sub-field under control of the control computer, as in Michael et al above. A pattern control section supplies signals to the spot shaping, blanking analog electronics unit. The shaped beam is vector positioned under the control of vector deflection apparatus. After each sub-field is completed the next sub-field is positioned by the large field deflection apparatus under the control of a large field deflection system. A pattern control section supplies signals to control a deflection system. The large field deflection apparatus may incorporate a Variable Axis Immersion Lens arrangement as described in Lananer et al U.S. Pat. No. 4,544,846. That arrangement permits the beam to be deflected farther from the axis of the electron beam column to provide a large range writing capability. The next sub-field is positioned orthogonally with respect to the direction of motion of the X-Y drive work table which supports the workpiece which usually has been a semiconductor wafer. However, at the edge of the writing field the exception is that the sub-field is positioned normally (or at right angles) with the direction of motion. The result of this sub-field positioning scheme in combination with a continuously moving work table is a continuously moving, boustrophedontic raster scan. The raster scan positioning of the sub-fields is controlled by the large field deflection control system containing electronic circuitry and pattern data from the pattern control section. A motion compensating signal from stage (work table) position measurement system is received by large field deflection system from the laser stage (work table) position measurement system. Measurement system preferably directs a pair of laser beams at two edges of the work table to measure table position as will be understood by those skilled in the art. Electronics in the system provide signals to the deflection apparatus to compensate for X, Y, and angular errors resulting from the continuous motion of the work table while sub-fields are being written.
U.S. Pat. No. 4,818,885 Davis et al for "Electron beam writing method and system using large range deflection in combination with a continuously moving table" shows a modified Hontas system.
U.S. Pat. No. 4,544,846 of Langner et al for "Variable Axis Immersion Lens Electron Beam Projection System" (commonly assigned) describes an E-beam system with a variable axis immersion lens (VAIL).
U.S. Pat. No. 3,900,736 of Michael et al for "Method and Apparatus for Positioning a Beam of Charged Particles" (commonly assigned) describes an E-beam exposure system with a computer driven correction system for use with a four corner registration system. The correction system operates dynamically to correct the deflection of an electron beam to minimize the deviation from desired alignment. Such alignment problems are caused by factors including the deviation of the position of the registration marks from their design positions. The Hontas system of E-beam exposure employed by Michael et al has heretofore employed a step-and-repeat method of performing the tasks of registration, writing and transporting of the workpiece to expose each field of a multi-field pattern on a workpiece. Heretofore, the Hontas step-and-repeat system has employed an A cycle, a B cycle and a C cycle. During the A cycle, the workpiece has been registered while the workpiece on the transporting table was at rest. Then the B cycle has followed, during which time the pattern to be exposed has been written by the E-beam, while the workpiece and the table still remained at rest. Finally, only in the C cycle, has the table supporting the workpiece moved along its time consuming trip to the next location for exposing the next field on the workpiece. The MEBES system provides continuous mechanical motion of the worktable supporting the workpiece, but it does not reregister the workpiece with respect to the E-beam during the process of exposing the entire workpiece. This has the advantage of speed since the reregistration steps are eliminated with the risk that the alignment of the workpiece and the E-beam deviates significantly from the desired alignment.
A Hontas electron beam exposure system utilizes a shaped-electron-beam which is vector scanned within a sub-field in combination with an electrical raster scan system for positioning sub-fields over a large rectangular area in combination with a continuously moving mechanical system for writing large patterns with a minimum of interruption. More particularly, it relates to a pattern writing system with a steered electron beam. This invention also provides a registration system using the electron beam to determine the target position accurately relative to the electron beam writing system. The system preferably employs writing of lithographic patterns with a shaped electron beam exposure system which minimizes the time wasted by workpiece positional requirements. Large lithographic patterns are written with subpatterns in sub-fields in a vector writing mode without interruption between successive sub-fields. This is made possible by continuously moving the workpiece in combination with the writing capability of a large rectangular writing field. The writing field contains a rectangular array of electronically positioned sub-fields which are written in a raster sequence. The large width of the writing field provided by the VAIL system reduces the number of mechanical scans required to write the pattern on the workpiece which further reduces the time required by workpiece positioning. The continuous velocity of the continuously moving workpiece during Y axis scans along a column on the wafer is corrected during writing to compensate for pattern density, maintaining an optimum workpiece position relative to the writing field. When patterns are being superimposed over previously written patterns, a means of registration is required since processing can cause workpiece distortions that are not detectable by position measurement systems. Accurate positioning of the overall workpiece relative to positional measurement systems is impractical due to thermal effects and other error sources. This system includes a registration field confined to local a reason the workpiece, which is larger than the writing field, which can be used for registration, without requiring a height-related change in focus and without requiring the mechanical system comprising the X-Y work table to change speed during the registration and reregistration of the various fields on a semiconductor wafer. The registration field can be larger than the writing field, because the quality requirements demanded from the shaped-electron-beam are less for detecting the locations of such registration marks at the various locations on the wafer.
In accordance with this invention, a large lithographic pattern is written as quickly as possible by writing sub-field patterns in a vector scan mode. The sub-fields are positioned with a large field deflection system in combination with a continuously moving workpiece. This rectangular array of sub-fields provides several advantages which reduce the total time required to expose the workpiece (i. e. wafer or mask). The large field deflection capability provides registration capability on sparsely located registration marks by using the beam as a probe to locate the target fields accurately relative to the deflection systems. The advantages over prior art are:
1. A wide strip of sub-fields is written for each pass of the wafer, thus minimizing the number of passes required and the time required for reversing the work table motion and repositioning. PA0 2. The large field deflection system provides the capability of positioning the sub-fields in a rectangular array thus providing a time buffer for dense and sparse patterns in the sub-fields. This capability reduces the velocity change requirements for the continuously moving work table. PA0 3. Registration capability provides a more precise superposition of the written pattern over existing lithography.
As electrons penetrate the substrate, they undergo a scattering process which will diffuse the beam and degrade the image. This is a well known phenomenon which can be dealt with during the conversion of the CAD data to tool data (also known as numerical control or NC data) via proximity correction algorithms.
These algorithms all assume that the resists being exposed have no dependence on temperature. In fact, however, this assumption is not warranted. As resist heats up, it becomes more sensitive (goes from unexposed to exposed at a lower threshold). This will lead to a loss.:of image acuity and possibly to calamitous defects in the pattern being exposed.
In the past when this happens one of the following actions was taken:
Redesign the offending pattern
Slow the exposure system down
Reduce the dose the resist is receiving
Each of these actions has major drawbacks. The first is usually not practical or achievable. The second reduces the throughput of an expensive exposure system, requiring the manufacturing plant to buy more systems. The third will leave other areas of the pattern unexposed.